mirakels it sounds like you are suffering from what will kill a lot of N900s, this is a common hardware problem and sadly will only get worse.

I have had two phones replaced under warranty because of this same problem. For me it seems to happen after six to eight months, but others have had theirs fail much sooner.

If you try a re-flash you will most likely get a CMT re-programming failure.

Try Power Searching for 'SIM failure' or 'SIM not recognized' to learn more.

Best thing to do is as jedi says and take it to a Nokia service centre. (I hope you are still in warranty)